Wednesday, May 17, 2017

Dellorto PHF PHM pump ramp profiles

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I have a Guzzi Sport 1100 in to attempt to tune, and in going through what had been changed and what effects those changes may have had, I found some specs for the pump ramps on the PHF and PHM slides.  So, the obvious thing to do was make a graph comparing them all.  It makes much more sense in a visual form for me.

The starting point is nominally the idle setting, but that's not a fixed thing so the actual "when stuff happens" numbers are a bit ish.

The slides are numbered with an XX/Y system, where the XX is the cutaway (in tenths of mm) and the Y is the pump ramp ID (it's just a number).  The cutaways I've seen are 50, 60 or 70, and the slides are /1 to /5.

The cutaway affects low slide lift mixture.  The smaller number has less of a cutaway at the rear edge of the slide, which increases the manifold vacuum transferred to the top of the needle jet (atomiser) and enrichens the mixture due to the greater pressure differential between the top of the needle jet and the fuel in the float bowl.

The maximum pump lever deflection on an old slide I had measured at 5.6mm, so that's the maximum number I used.  The slide travel from closed to wide open is about 1mm less that the carb bore diameter, as at idle the opening under the front of the slide is about 1mm.  Ish.

The Sport 1100 had a /5 std, this one has /3.  Don't know if that's a good or bad thing.  Lots of the 41mm Malossi converted PHM 40 don't have pumps at all, and they're a "racing" carb.  And a trick when you're dyno tuning Dellortos is to pull the pump lever so you don't get any pump shot influence in your readings.  So you can do without them if you're not slamming the throttle open in an aggressive fashion (ie, doing roll on wheelies).  Where on earth is the fun in that you may well ask.  I would.



As an aside of some relevance, I'll add this from the Moto Guzzi Lemans 1000 manual.  It's just so delightfully delusional, and always makes me smile.




Dream on, my man, dream on.

1 comment:

Alasdair Cameron said...

Classic.
That reminds me of an ancient Haynes Carb tuning book i once saw.
With regard to setting the main jets, they suggested finding a long flat straight that could be used to do a WOT run before abruptly killing the engine and pulling the plugs to check the colour.
"This procedure may incur the displeasure of the local constabulary" was the warning.
Nice.
Good old days? Not so sure....